Hollow carbon arc discharge



Oct. 11, 1960 J. 5. LUCE I 2,9

HOLLOW CARBON ARC DISCHARGE Filed Aug. 1 4, 1959 TO VACUUM ION SOURCE TOVACUUM TO VACUUM GAS SOURCE INVENTOR. John S. Luce BY i ATTORNEY Un tedStates. Patfi f HOLLOW CARBON ARC DISCHARGE John S. Luce, Oak Ridge,Tenn., assignor to the United States of America as represented by theUnited States Atomic Energy Commission Filed Aug. 14, 1959, Ser. No.833,897

Claims. (Cl. 313-153) against the instreaming of low-energy neutralparticles into a plasma formed within the hollow discharge. Reduction ofincoming neutrals reduces the number of charge exchange reactions andthereby increases the probability of creating and maintaining adequatereaction temperatures to produce neutrons from a thermonuclear plasma.

This invention is an improvement over the carbon arc discharge set forthin my co-pending application, Serial No. 738,242, filed May 27, 1958,now Patent No. 2,920,234, issued January 5, 1960. The arc discharge setforth in that application is very efficient as a dissociating and/ orionizing mechanism, but in order to achieve and maintain adequatethermonuclear reaction temperatures, it would be necessary to trap avery large quantity of high energy ions to ionize the neutrals in thesystem. This phenomenon is commonly called burnout.

One major problem associated with establishing and operation of aneutron producing or thermonuclear plasma by trapping of atomic ionsinside a magnetic mirror field is the effect of cooling of energeticions by collision with low energy neutral particles. The magnitude ofthis problem of charge exchange is such that a thermonuclear plasmacannot be obtained unless burnout of the residual neutral gas occurs, asset forth in the application of Albert Simon, Serial No. 732,770,

filed April 28, 1958. h

It is recognized that when energetic particles strike the walls of anycontainment vessel, there will be low energy neutral particles producedthat will degrade the temperature of the plasma. In addition, similarcooling neutrals will arise from outgassing and other sources within thevessel and thus will cause untenable charge exchange and ionizationlosses. Many means for reducing charge exchange in the plasma have beensuggested. These include evaporating a getter over the entire innersurface of the machine; burying ions in plates with means provided forrenewing the surface thereof; and providing semi-porous liners to permitthe escape of these particles but retard the reverse flow into themachine. None of these means, however, appear as effective as the meansset forth herein to reduce charge exchange.

Another problem associated withignition of a thermonuclear plasma by theLuce method of dissociation of molecular ions to form atomic ions bypassing the former I through an energetic arc discharge is thesubsequent capture of electrons by the hot ions, forming neutrals whichare not confined and are lost to the system; 'The carbon ions formed incarbon discharges where the anode is within the mirror region arepredominantly singly ionized (C+) ions, with fewer multiply ionizedions. The anode 2,956,195 Patented oer, 1 1 .1960

cannot, in solid arcs, be placed any substantial distance behind themirror. without the arc becoming serpentine and kinking due to theinteraction of the mirror field and the circular field around the arc.

An unexpected advantage of the hollow. carbonarc herein described isthat it will remain, stable even when the anode is placed .a longdistance behind the mirror region. With such spacing, I have foundi(l)that substantially 'fewericarbon ions enter the plasma region tocontaminate it, and (2) that the ratio of multiply ionized to singlyionized carbon ions is greatly increased. 'The highly ionized carbonatoms compete forelectrons with the plasma gas atoms, so actually helpto reduce charge exchange losses.. Thus the hollow arc can operate withthe arc anode substantially distant behind amirror coil to achieve .evengreater protection for the plasma it surrounds. I J 7 Some advantagesofa hollow arc discharge have been set 'forth in other applications. Oneexample of such discharges is set forth in my co-pending' application,Serial No. 748,771, filed July 15, 1958, now Patent No. 2,927,232,issuedMarch -1, 1960. The discharges set forth in that application aresustained by gas fed to the discharges. .The devices for producing suchdischarges,

as .a .barrierfor low. energy neutral particles attempting to enter thehollow interior ofthe discharge, and to pro vide means for assisting inremoving excess gas particles fromtheapparatus.

- It is .another object of this invention to provide a hollow, gas-fedcarbon arcdischarge in an evacuated container and in a-strong confiningmagnetic field for dis 'sociating and ionizing an injected molecularionbeam, to

form a trapped plasma of-ener getic ions and electrons withintheregionboundedby the discharge, and to reduce ion losses from the plasmadue to the charge ex change process, whereby the density of the plasmamay be increased sufilciently to reach thermonuclear temperatures,producing neutrons. j h V V -It is still another object of thisinvention to provide' a hollow carbon arc discharge which is effectiveas an ion pump for low-energy ions formed Within the discharge.

These and other objects and advantages of this invention will becomeapparent from'a consideration of the following detailed specificationand the accompanying drawing, wherein the single figure is across-sectional view of a mirror-type device for accomplishing the aboveobjects. Q1

The above objects have been accomplished in the present invention byinitiating and sustaining'an energetic hollow carbon arc dischargebetween a thin hollow carbon cathode and a hollow carbon anode in anevacuated enclosure and within a strong confining magnetic fieldprovided by two spaced magnetic mirror coils. The electrodes arepreferably positioned with the axes thereof alongthe axis of magneticsymmetry.' 'The electrode faces are positioned in regions ofcompressedmagnetic field so as to form an are which is enlarged indiameter midwaybetween the electrodes due to the .bowing out of the magnetic fieldlines in this region; .One method 0f forming a thermonuclear-Plasma of.trapped atomic hollow 'arc discharge is to inject a beam of molecularions at near grazing angle to the inside region of the arc wall, where aportion of the molecular ions are dissociated and/or ionized by thedischarge and thus form a trapped plasma of atomic ions and electronswithin the volume of the arc discharge. The are wall then serves as ashield or barrier to the cold neutral particles formed outside of thehollow arc.

Another method for forming a thermonuclear plasma of trapped atomic ionsand electrons within the volume established by the hollow arc dischargewould be to provide a gas fed arc discharge in axial alignment with andin the center of the hollow arc discharge and to provide a potentialgradient between the discharges in a manner as set forth in my copendingapplication, Serial No. 790,- 031, filed January 29, 1959. In thisarrangement, ions would be accelerated from the more positive arc towardthe negative arc and electrons from the negative arc to the morepositive arc and they would be magnetically trapped between the arcs toform a plasma of energetic ions and electrons.

The use of carbon electrodes in the present invention has at least twomajor advantages over discharges using other materials. One of theseadvantages is that due to the low atomic number of carbon, the arcdischarge formed between such electrodes is well defined and producesless scattering of ions from the discharge. By keeping this scatteringat a minimum, the ion losses from the plasma are reduced. Anotheradvantage is economy. Carbon pieces and particles are torn from theelectrodes during arc operation, and these pieces and particles,together with the carbon ions, absorb gas to such an extent that it ispossible to feed a large amount of gas to the discharge and still notrequire as large a pumping system as would otherwise be required.

Refer now to the single figure in the drawing Which illustrates oneembodiment in which the principles of this invention may be carried out.A hollow carbon cathode is supported within and afiixed to a tubularmember 16. Member 16 is enclosed partly by an insulating. sleeve member17 which has a flange portion. This flange portion is clamped againstone end wall of housing 1 by a clamping ring 35 With retaining boltstherethrough which areafiixed to the end wall of housing 1. Ahollowcarbon anode 11 is supported within and affixed to a tubularmember 19. Member 19 is enclosed partly by an insulating sleeve member20 which is provided with a flange portion. This flange portion isclamped against the other end wall of housing 1 by a clamping ring 36with retaining bolts therethrough which are afiixed to said end wall ofhousing 1.

A hollow cup-shape bafide 12 is disposed within and spaced from cathode10., and extends beyond the leading edge of cathode 10. Baffle 12 issupported by an insulating rod 14, and rod 14 is in turn supported by amember which is fitted within tubular member 16. A hollow sleeve-likebaflle 13 is closely fitted within and supported by hollow anode 11, andit extends beyond the leading edge of anode 11. Baffles 12v and 13 areextended beyond the leading edges of cathode 10 and anode 11,respectively, to prevent carbon particles and carbon pieces, which areproduced at the electrode edges, from reaching the inner region of thehollow are 31 formed between the cathode and anode. Bafiie 12 must beinsulated so as to prevent it from serving as the cathode for the aredischarge. Insulating rod 14 is provided with a central conduit 40, towhich is connected a gas feed tube,38, which in turn is connected to asource of feed gas 37. Hollow baflle 12 has a plurality of openings 39adjacent to the leading edge of cathode 10 so that gas may be fed to thearc discharge formed between cathode 10 and anode 11.

The gas is used not only in assisting in the formation of the discharge,but in helping to sustain the discharge and reduce scattering from thedischarge.

The container 1 is divided into three separate chamthe conductancebetween the chamber 33 and the higher I pressure end chambers 32 and 34.The annular passageway between the cathode 10 and baflle 12 is providedto permit escape of the ions produced in the arc and thus permit the arcto more effectively act as anion pump. These ions may then escapethrough a plurality of openings 18 in tubular member 16. An additionalportion of the ions formed in the arc leave the inner region 33 throughan annulus between arc discharge 31 and the outer bafiles 21.

An annular magnetic mirror coil 23 is disposed adjacent to andsurrounding cathode 10 and baffies 21 in such a position that the faceof cathode 10 is in a region of compressed magnetic field. An annularmagnetic mirror coil 24 is disposed adjacent to and surrounding anode 11and baffles 22 in such a position that the face of anode 11 is in aregion of compressed magnetic field. The cathode 10 and anode 11 arepositioned withthe axes thereof along the axis of magnetic symmetry.Thus the arc discharge 31 formed between cathode 10 and anode 11 is asymmetrical hollow are which follows the 'magnetic field lines, and thearc is enlarged in diameter midway between the electrodes due to thebowing out of the magnetic field lines in this region.

Chamber 33 is connected to a vacuum pump, not shown, through aconnection 5 and opening 8 therein. Chamber 32 is connected to a vacuumpump, not shown, through a connection 3 and opening 6 therein. Chamber34 is also connected to a vacuum pump, not shown, through a connection 4and opening 7 therein.

Cathode 10 is electrically connected through tubular member 16 and lead27 to one side of a source of a variable D.C. voltage 25. The other sideof source 25 is connected by a lead 26, and tubular member 19'to anode11. Source 25 and gas from source 37 are used for initiating and helpingto sustain the arc discharge between cathode 10 and anode 11. Thissource 25 and/or any other conventional means may be employed forstriking the arc discharge as described in my 'co-pending applicationSerial No. 728,754, filed April 15, 1958. Such other means includeheating the cathode and anode electrodes to outgas said electrodes,providing a movable auxiliary electrode adjacent to the cathode andapplying a RF. voltage across said auxiliary electrodeand cathode untilan arc is struck and then removing the auxiliary electrode, or by makingthe position of the anode adjustable with respect to the cathode untilan arc is struck and then varying the relative position of the anode andcathode to a desired operating position.

An energetic, high current arc discharge in a confining magnetic fieldis an efficient mechanism for dissociating and/or ionizing a molecularion beam to form a magnetically trapped plasma of energetic ions andelectrons as set forth in my aforementioned co-pendingapplication SerialNo. 738,242, now Patent No. 2,920,234, issued January 5, 1960. Thatapplication, however, does not disclose the use of a hollow arcdischarge for confining the thus formed plasma within the hollow regionof a hollow arc discharge. The hollow discharge in the present inventionin conjunction with the magnetic field is used as a shield or' barrierfor the low energy neutral particles, formed outside the discharge, fromentering the plasma region, and thus to more readily effect burnout ofthe neutral particles in the plasma region within the discharge. Anysuch low energy neutral particle,whatever its origin, is ionized in thedischarge upon contact therewith and as such is pumped along thedischarge and carried out of the reaction zone of the device.

One typical wayof igniting a high energy plasma in the device set forthabove, is to inject a beam 30 of molecular ions from a source 28 andthrough an accelerator tube 29 into the path of the discharge 31 at neargrazing angle to the inside wall of the discharge, where a portion ofthe molecular ions are dissociated and/or ionized to form a magneticallytrapped ionized plasma of atomic ions and electrons in a manner as setforth in my aforementioned co-pending application Serial No. 728,754.The accelerator tube 29, referred to above, is energized by a suitablehigh voltage generator. A suitable high current source of molecular ionsfrom source 28, may be provided by apparatus such as set forth on page18 of Nucleonics, vol. 9 (3), 1951; Review of Scientific Instruments,vol. 24, p. 394, 1953, for example; or that described by Von Ardenne,Tabellen der Elektronephysik, Ionenphysik, and Ubermikroskopie, VEBDeutscher Verlag der Wiscenschaften, Berlin 1956 (Duo- Plasmatron), orby means such as disclosed in my copending application Serial No.833,895, filed August 14, 1959, now Patent No. 2,933,630, issued April19, 1960. Accelerator tube 29 is fitted in a member 9 in outside wall 1,and extends into the interior of chamber 33 and fitted within liner 2,as shown. The molecular ions from source 28 may be D or D for example,and have an accelerated energy of about 600 kev., for example. Themagnetically trapped atomic ions will then have an energy of about 300kev. if D is used, or 200 kev. if D is used.

In one embodiment of the device, described above, the cathode 10 wasabout 4.5 inches in diameter and had a wall thickness of about 0.062inch. The annulus between the cathode 10 and the inner baflle 12 wasabout 0.25 inch, and the annulus between the outer battles 21 and thedischarge 31 and between the outer baflles 22 and the discharge 31 wasabout 0.25 to 0.30 inch. The spacing between the anode 11 and cathode 10was about 42 inches. Arcs obtained with this geometry have been about5.7 inches in diameter at the mid-plane between the magnetic mirrorsprovided by coils 23 and 24. The current in such arcs ranges between3500 and 4000 amperes, with a voltage drop between the electrodes ofabout 160 volts. 5X10"- mm. Hg, and the vacuum in chambers 32 and 34 wasabout 5 10 mm. Hg.

The coils 23 and 24 each provide a magnetic mirror field, the value ofwhich is as high as can be obtained, say 3000 to 10,000 gauss.

In operation of the apparatus having the parameters set forth above, ahollow carbon arc discharge 31 is initiated between cathode 10 and anode11 by feeding gas through openings 39 in baffle 12 and byapplying any ofthe arc initiating assisting means set forth above. After the arc isstruck, the potential source 25 is adjusted to a desired operatinglevel, which will provide a voltage drop between the electrodes of about160 volts. The pressure in chamber 33 is adjusted to a vacuum pressureof about 5 l0 mm. Hg, and the pressures in chambers 32 and 34 areadjusted to a pressure of about 5 10-- mm. Hg. The coils 23 and 24 areenergized to provide an average field strength of about 3500 gauss. Thedischarge 31 is sustained partially by gas particles in the dischargeand partly by carbon ions produced as a result of electron bombardmentof the anode and/or other ion-forming means. Vaporized carbon particlesfrom the cathode absorb gas and thus produce a high pumping capacity forneutral particles ionized in the discharge. The are current was about3500 amperes for the above operating parameters. Very stable arcs areobtained when the power input is about 35 kw., or greater, percircumferential inch of the are at the cathode.

Due to the severe erosion of the cathode, it may be desirable tosubstitute a composite cathode, e.g., a tungsten-graphite electrode forthe carbon cathode. Ions for the arc would be supplied, as before,primarily from the The vacuum in chamber 33 was about,

obtainable pressure.

It has been determined that by regulating the rate of gas feed to thedischarge through the openings 39 in baffle 12, it is possible toprovide a generous quantityof gas particles to the discharge withouthaving to provide a large pumping system for removing excess gasparticles since they are, to a large extent, removed by being absorbedby carbon pieces and particles released from the electrodes during arcoperation. It is estimated that the number of gas particles may bevaried from 50 to 75 percent of the total number of gas particles andcarbon particles present during arc operation, while the number ofcarbon particles would correspondingly vary from 50 to 25 percent ofsaid total. These percentages may readily be determined byspectrographic analysis, and the ratio of gas to carbon particles may becontrolled by regulating the feed rate of the gas fed to the discharge.Thus, it can be seen that gas to establish and sustain the discharge maybe fed to the discharge in a generous quantity and the benefits of thepresence of such gas in the system, as discussed above, may be realizedwithout the use of a large pumping system for removing excess gasparticles, because of the gas absorbing properties of the carbonparticles released from the electrodes.

In one way of igniting a highly energetic plasma in the above device, abeam 30 of energetic molecular ions, D or 133 for example, is injectedfrom a source 28 and through accelerator tube 29, with an energy ofabout 600 kev., into the inside wall region of the hollow arc discharge31, where a portion of the molecular ions are dissociated and/or ionizedinto atomic ions, electrons, and neutrals. The atomic ions thus formedare magnetically trapped by the magnetic field, provided by coils 23 and24, in a manner as set forth in my aforementioned co-pending applicationSerial No. 728,754. The highly energetic plasma thus formed may beincreased in density to a degree sufficient to thereby produce athermothus greatly reducing the charge-exchange process within theplasma region. Losses are further reduced by reducing scattering fromthe discharge as discussed above, and by maintaining the pressure'inchamber 33 at the lowest Any low-energy neutral particles that strikethe discharge 31 are immediately ionized by the discharge and pumped outof the system along the discharge as discussed above. As set forthabove, a carbon arc discharge is a very eflicient ion pump.

Where the anode is moved outside the mirror, the arc spacing may be 73",for example, with the same other arc parameters above recited.

Thus, it can be seen that I have provided an improved arc dischargewhich is useful as a dissociating and/or ionizing mechanism forforming-a thermonuclear plasma. The substantial reduction of scatteringand. the charge exchange process by use of the hollow carbon, gas-fed,arc discharge, as discussed above, will permit a more rapid formation ofa thermonuclear plasma, since burm out of the low energy neutralparticles in the system may be accomplished in a shorter time.

The use of the carbon arc to ignite an energetic plasma of atomic ionsand electrons has certain applications in the particle accelerator artalso. For example, the breakup of D ions and trapping of protons fromapoint from the point of dissociation may be utilized as a method ofinjecting protons into a proton synchrotron.

When used for dissociation of molecular ions, the high current are canalso be 'used for producing large well defined neutral beams. Since thecross section for dissociation decreases slowly as the energy increases,this process can be used at any desired energy. These neutral particlescan be injected into accelerators and then converted into ions.

It has been determined that for the above operating conditions, there isat least a 25 percent breakup of the molecular ions into atomic ions.This percentage increases linearly as the arc current is increased.

This invention has been described by way of illustration rather thanlimitation, and it should be apparent that the invention is equallyapplicable in fields other than those described.

What is claimed is:

1. A device for producing an energetic, hollow, carbon arc discharge,comprising a container, said container being provided with a centralchamber and two end chambers; an elongated, hollow carbon cathodeprovided with an annular face and mounted in one of said end chambersand extending to within said central chamber; a cup-shaped baffiemounted within, spaced from, and extending beyond the face of saidcathode, said bafile being provided with a plurality of radial aperturespositioned just beyond the face of said cathode; an elongated, hollowcarbon anode provided with an annular face and mounted in the other endchamber and extending to Within said central chamber, the face of saidanode being in confronting relation tothe face of said cathode and beingwidely spaced therefrom, said cathode and said anode having a commonaxis; a sleeve-like bafile closely fitted within said anode andextending beyond the face 'of said anode; a first plurality of annularbafiles disposed just beyond the face of said cathode and being affixedto said central chamber; a second plurality of annular baffles disposedjust beyond the face of said anode and being affixed to said centralchamber; a first annular magnetic mirror coil disposed about saidcathodeand said first plurality of annular baffles to provide a first,constricted magnetic field having field lines with an axis in alignmentwith said common axis; a second annular magnetic mirror coil disposedabout said anode and said second plurality of annular baffles to providea second constricted magnetic field having field lines with an axis inalignment with said common axis; a source of feed gas; means connectedbetween said source and the interior of said cup-shaped bafile forfeeding gas thereto and then through said apertures at a controlledrate; means connected to each of said chambers for evacuating saidchambers to selected pressures; and means connected between said anodeand said cathode for initiating and sustaining a hollow, gas-fed, arcdischarge therebetween which follows the magnetic field lines, saiddischarge being sustained partly by carbon particles released from saidanode and cathode and partly by gas particles fed to said dischargethrough said apertures when said discharge is operating, the ratio ofgas particles to carbon particles being a function of the rate of gasflow to said discharge, said extensions of said bafiies disposed withinsaid anode and said cathode preventing said carbon particles fromentering the interior region of said discharge, each of said firstplurality of annular baffles defining an annular space between each ofsaid first baffles and said discharge, the discharge after being formedserving to pump the ions formed in the discharge out of the centralchamber into said one end chamber through said annular spaces.

2. The device set forth in claim 1, wherein the magnetic field of eachof said mirror coils is maintained at an average flux density of about3500' gauss, the pressure in said central chamber is established at avalue of about 1O* mm. Hg, the pressure in each of said end chamhers isestablished at a value of about 5 10 mm. Hg, said means for initiatingand sustaining said discharge including a source of voltage at anoperating value of about 160 volts to produce an arc discharge currentof about 3500 amperes.

3. The device set forth in claim 1, and further including a source ofmolecular ions; and means connected to said ion source for acceleratingand injecting a beam of said ions with a selected energy and densityinto the inside wall region of said discharge where a portion of saidmolecular ions are dissociated and ionized to form a highly energeticplasma of magnetically trapped electrons and atomic ions within theregion bounded by said hollow discharge, whereby the charge exchangebetween cold neutral particles and the ions of said plasma is greatlyreduced by the natural barrier of said discharge to cold neutralparticles formed outside said discharge, in that said neutral particlesare ionized by said discharge and pumped out of said central chamber bysaid discharge through said annular spaces between said first luralityof annular baflles and said discharge.

4. A device for producing an energetic, hollow, carbon arc discharge andfor producing and shielding a plasma, comprising a container; a pair ofwidely spaced, hollow, carbon electrodes mounted on a common axis andwithin said container, each of said electrodes being provided with anannular face, said annular faces being in confronting relation withinsaid container; a first annular electromagnetic mirror coil disposedaround one of said electrodes; a second annular electromagnetic mirrorcoil disposed around the other of said electrodes, said mirror coilsproviding magnetic field lines which are oriented in a directionparallel to said common axis and which are constricted about saidelectrodes; evacuating means connected to said container forestablishing a selected pressure within said container; a source of gas;means connected to said source of gas for feeding gas at a controlledrate to the annular face of one of said electrodes; means connectedacross said electrodes for initiating and sustaining a gas-fed, hollow,carbon are discharge therebetween, said discharge being sustained partlyby carbon particles released from said electrodes and partly by gasparticles fed to said discharge during operation of said discharge, theratio of gas'particles to carbon particles being a function of the rateof gas flow to said discharge, said discharge following said 'magneticfield lines; means disposed within and extending beyond the annularfaces of said electrodes for preventing said carbon particles fromentering the interior region of said hollow discharge; a source ofmolecular ions; and means connected to said ion source for acceleratingand injecting a beam of said ions into the inside wall region of saiddischarge, where a portion of said ions are dissociated and ionized toform a highly energetic plasma of magnetically trapped electrons andatomic ions within said interior region, thereby isolating said plasmafrom cold neutral particles formed outside the boundary of saiddischarge, in that when said neutral particles contact said dischargethey are ionized thereby and pumped by said discharge away from saidplasma, such that said plasma may acquire sufiicient density to producethermonuclear neutrons.

5. The device set forth in claim 4, wherein said coils provide amagnetic field having an average flux density of about 3500 gauss, thepressure in said container is established at about 5X10 mm. Hg, saidmeans for initiating and sustaining said discharge including a source ofvoltage at an operating value of about volts to produce an arc dischargecurrent of about 3500 amperes; said molecular beam comprising D ions,and the energy of said molecular beam is maintained at about 600 kev.

References Cited in the file of this patent UNITED STATES PATENTS2,675,470 Wideroe Apr. 13, 1954 2,719,240 Walker Sept. 27, 19552,855,537 Mendel Oct. 7, 1958 2,919,370 Giannini Dec. 29, 1959

